1. Field of the Invention
This invention relates to an optical color-separation system for use in a color television camera, and more particularly to such an optical color-separation system having means to project a beam of bias light to each of the camera tubes used in the color television camera.
2. Description of the Prior Art
FIG. 1 shows the basic structure of a relay type color-separation system for use in a color television camera. In the drawing the light rays from an object pass through a taking lens 110 and are focused on a field lens 120 to form an image. The light rays from the so-formed image are collimated by a first relay lens system 140, and the collimated light rays are separated into different color components by dichroic mirrors 151 and 152. Selected color component beams are directed along different paths by mirrors 153 and 154. Thus, three different color component beams are projected through second relay lens systems 161, 162 and 163 to three associated camera tubes and images of different color are formed in the camera tubes.
With a view to improving the "after image" effect of an image to be taken with a camera tube, it has been proposed to optically condition the camera tube by projecting thereon not only the light carrying the image information but also an additional illuminating light of given constant intensity. In practice, as shown in FIG. 1, a half mirror 131 is positioned between the field lens 120 and the first relay lens system 140, and a diffusing plate 132 is placed at a distance from the half mirror 131 equal to the distance from the half mirror 131 to the field lens 120. A beam of light from a light source 135 is projected onto the diffusing plate 132 so that a bias light is added to the beam of light carrying image information. Alternatively a half mirror is put in the first relay lens system 140 as a means for introducing a beam of bias light into the image forming optical path. However, the optical distances from the first relay lens system 140 to the second relay lens systems 161, 162 and 163 are not equal, and therefore different parabolic shadings inevitably result. With a view to reducing these parabolic shadings, a correction filter 133 is used to distribute the bias light on the diffusing plate 132 in a counter-parabolic shape to produce parabolic shadings opposite to those which would otherwise result. However, it is difficult to completely eliminate the parabolic shadings in this way because they differ somewhat in the red, green and blue channels. Also disadvantageously, different color shadings result from the different angles of light incidence on the dichroic mirrors 151 and 152, and these color shadings are also difficult to reduce. Thus, uniform illumination of all camera tubes with the bias light is practically impossible with the bias light introducing system mentioned above, and it is often necessary to use an electric circuit for the purpose of compensation. The conventional bias light introducing system uses a half mirror as indicated at 131, and disadvantageously a certain amount of light is lost both in the image-forming channel and the bias light introducing channel. Also disadvantageously, a color compensating filter is inevitably used to provide a proper amount of light to each of the optical channels and the color temperature of the electric bulb must be controlled at a given constant value.